In the active matrix type LCD device using thin film transistors (TFTs) as switching elements, a liquid crystal layer is sandwiched between two transparent substrates, and its display is performed by respectively controlling an electric field applied to the liquid crystal in every pixel arranged in a matrix pattern.
Driving modes of the liquid crystal are roughly divided into two modes. One is a vertical electric field mode and the other is a lateral electric field mode or so called an In Plane Switching (IPS) mode. The vertical electric field mode is a driving mode which drives the liquid crystal by applying a vertical electric field to a principal surface of a glass substrate. The IPS mode drives the liquid crystal by an electric field parallel with the substrates. The IPS mode has an advantage that angle of visibility dependence is small, because only a short axial direction of liquid crystal molecule is observed. The present invention relates to the LCD device of this IPS mode.
In a basic structure of the active matrix type LCD device with the IPS mode, each pixel of a TFT substrate includes a pixel electrode, a common electrode, a TFT, a common line, a video signal line, and a scanning signal line, and they have following functions.
The pixel electrode is applied with a desired electric potential so as to generate the lateral electric field for driving the liquid crystal between the pixel electrode and the common electrode. The common electrode is applied with a reference potential. Each TFT operates as a switch element for connecting/cutting between the video signal line and the pixel electrode. The common line is provided to supply a common potential to the common electrode. The video signal line is provided to supply a video signal voltage to be written in each pixel. The scanning signal line is provided to supply an electric potential to a gate electrode of the TFT so as to control a switching state of on or off of the TFT.
In such IPS mode LCD device, it is important firstly to make the electric potential of the common electrode uniform in order to achieve a stable display for an entire display area. To this end, the resistance of the common line which supplies the electric potential to the common electrode needs to be made low.
Second, it is important for the electric field applied between the common electrode and the pixel electrode so as not to be influenced by potentiodynamic of the video signal line. To this end, a widely used method is that the common line is overlapped on the video signal line with an insulating film interposed between them so as to shield the electric field of the video signal line.
In this method, however, because currents of charge and discharge flow through a parasitic capacitance formed between the video signal line and the common line, its power consumption is increased. In order to suppress the power consumption small, this parasitic capacitance needs to be decreased. For example, the technology which realizes these requests is disclosed in Japanese Patent Application Laid-Open No. 1999-2828 (Patent Document 1: see Paragraphs 30 through 32). This technology will be described with reference to the drawing in the followings.
FIG. 13A is a plan view of a schematic diagram showing a pixel in the technology of the Patent Document 1, and FIG. 13B is a sectional view taken along a dashed line VI-VI shown in FIG. 13A.
The scanning signal line 102 and the video signal line 101 are extending along a horizontal direction and a vertical direction on a drawing sheet of FIG. 13A, respectively, to form a matrix pattern on a TFT substrate. Each TFT 108 is provided at a position corresponding to an intersection of each matrix pattern so that each grid section forms each pixel. One source/drain electrode 110a of the TFT 108 is connected to the video signal line 101, and the other source/drain electrode 110b is connected to the pixel electrode 107. The pixel electrode 107 is formed to have a comb-shaped pattern with its each comb-tooth extending into the pixel area.
As shown in FIG. 13B, a thick transparent insulating film 118 is formed on the video signal line 101 and the pixel electrode 107 (and the scanning signal line 102 which is not shown). The common line 103 in a grid-shaped pattern is formed on the insulating film 118. The common electrode 106 is formed to have a comb-shaped pattern with its each comb-tooth extending into the pixel area such that each comb-tooth is disposed between the pixel electrodes 107 with a predetermined interval. This interval is an aperture on the TFT substrate.
This technology is most notable in the following two points.
First, by forming the common line 103 to have a grid-shaped pattern, its resistance can be reduced more remarkably than the case where the stripe-grid pattern is used. Thus a line width of the common line 103 can be reduced and results in achieving increased aperture ratio. This advantage becomes more remarkable by using a low-resistance metal layer for the wiring. The Patent Document 1 indicates to use one of Al, Cr, Ti, Mo, and a laminated layer of these metals.
Second, by increasing the thickness of the insulating film 118 between the common line 103 and both of the video signal line 101 and the scanning signal line 102, its parasitic capacitance decreases. For this reason, the charge and discharge currents which flow during voltage variation of the video signal line can be reduced. In order to reduce the parasitic capacitance, it is also still effective to make the dielectric constant of the insulating film small, as disclosed in WO 1998-047044 (Patent Document 2: see Page 7, line 37 through page 8, line 3).
However, there is a problem with the LCD device disclosed in the Patent Document 1. When the common line is made of metal layer, its surface reflects an ambient light, and so-called glare occurs. For this reason, a light shielding layer has to be provided on the opposed substrate and when the light shielding layer is provided, its aperture ratio decreases by that amount. In addition, its manufacturing step should allow a margin to the overlapping conformation difference between the TFT substrate and the opposed substrate, and concerning the margin in both of a longitudinal direction (vertical direction) and a transversal direction (horizontal direction), its aperture ratios is further decreased.